accurate hydrometer this stem is graduated, and serves to shew thedensity of the fluid, by the depth to which it sinks ; as the heavierfluids will buoy up the instrument more than such as are lighter.In this way, however, it is clear, that the stem must be-comparu-tivcly thick, in order to possess any extensive range; for theweight of vitriolic ether is not ecpjal to three-fourths of the samebulk of water, and therefore such an hydrometer, intended to ex-hibit the comparative densities of these fluids, must have its stentequal in bulk to more than one-fourth of the whole instrument.If this bulk be given chiefly in thickness, the smaller differencesof density will not be. perceptible, and it cannot, with any conve-nience, be given in length. To remedy this imperfection, variouscontrivances have been proposed, for the most part grounded onfhe consideration, that a change in the ballast, or weight employedto sink the ball, would so lav change the instrument, that thesame short range of gradations, on a slender stem, which wereemployed to exhibit the densities of ardent spirit, might be em-ployed in experiments upon water. Some have adjusted weightsto be screwed upon the lower stem, and others, with more neatnessand accuracy, have adjusted them to be slipped upon the extre-mity of the upper stem. But the method of Fahrenheit appearsfo be on all accounts the simplest and most accurate. '1 be hy-drometer of Fahrenheit consists of a hollow ball, with a counter-poise below, and a very slender stem above, terminating in a smalldish. The middle, or half-length of the stem, is distinguishedby a fine line across. In this instrument every division of thestem is rejected, and it is immersed in all experiments to the mid-dle of the stem, by placing proper weights in the little dish above.Then as the part immersed is constantly of the same magnitude,and the whole weight of the hydrometer is known ; this' lastweight, added to the weights in the dish, will be equal to theweight of fluid displaced by the instrument, as all writers on hy-drostatics prove. And accordingly the specific gravities for thecommon form of the tables will be bad by this proportion. Asthe whole weight of the hydrometer and its load, when adjusted indistilled water, is to the number 1,000, &c. so is the whole weight,

when adjusted in any other fluid, to the number expressing its spe-cific. gravity. In this -■ - ' *

....... ..at changing the weights. .... -•_—

represented in fig. 1, Plate X.CV. It is chiefly employed ineevtaining the specific gravity of spirits. See Specific Gravity. In using the hydrometer, its stem should always be presseidown lower than the point at which it will ultimately rest, that b;being wetted it may occasion no resistance to the fluid. The instvumeiit itself should be of as regular a shape and with as few in

equalities as possible, tliat all impediments to Us motions may bavoided.

HYDROMETRY, [Mwg and p-Tgsv,] the art of measuring thextent of water.

RYDROMPHALUS, water, and ofj.iuo.o;, the navel,3tumour in the navel, containing water.

HYDROPHIT-US, a genus of insects, order, coleoptera. Antemia; clavate, club perfoliate j. feelers, four, filiform.; hind-leg

formed for swimming, fringed on the inner side. They inhabitponds and stagnant waters, and swim with great dexterity. Thereare about thirty species. The-most remarkable European speciesis the II. piceus, water-clock, frequently seen in our ponds. Thelarva: of the hydrophihis are supposed to remain about two years-before they change into pupa: or chrysalides. When the larva: isarrived at its full growth, it secretes itself in the bank of the waterit inhabits, and having formed a convenient cavity or cell, lie*dormant for some time, after w hich it divests itself of its skin, andappears in the form of a chrysalis, in which stale having continuedfor seme time longer, it again delivers itself from its exuviiv, andappears in its complete or beetle form. When first disengagedfrom the skin of the chrysalis, it is of a pale colour, and very ten-der : but in the space of a few hours the elytra, or wing-cases, ac-quire a degree of strength and colour, which gradually grows-more and more intense, till the animal, finding itself sufficientlystrong, comes forth from its retreat, and commits itself in its newform to the waters. The male is distinguished from the female bythe structure of the fore-legs, which are furnished, near the setting,on the feet, with a sort of horny concave flap or shield ; the legs otthe females being destitute of this part. The female of the hy-drophihis piceus affords an example of a faculty which seems to beexercised by no other coleopterous insect, viz, that of spinning a-kind of web or flatfish circular case of silk, which it leaves floatingon the water, and in which it deposits its eggs, This case is ter-minated on its upper surface by a lengthened conical process re-sembling a born, of a brown colour, and of a- much stronger ordenser nature than the case itself, which is white-. The younglarva-, as soon as hatched, make their escape from the envelope-ment of the case, and commit themselves to the water.

HYDROPHOBIA, dread of water. A symptom

of madness caused by the bite of a mad animal.

Hydrophobia has likewise been sometimes found to-take placein violent inflammations of the stomach, and in hysteric tits. SeeMedicine.

HYDROPI1YLAX, in botany, a genus of the monogyniaorder, and tetrandria class of plants. Calyx tetrapartite ; corollafunnel-shaped; fruit two-edged, one-sceded. It has one species,i a native of the East-Indies.

IIY DROPH YLLUM, Water-Leaf ; a genus of the mono-gynia order, and pentandria class of plants. Corolla campanutat-ed, with five melliferous longitudinal stria on the inside; stigmabifid ; capsule globose, bivaived. It has two species.

HYDROSCOPE, ['vSwf and <txo-ko, 3 an instrument ancientlyused for the measuring of time. It was a kind of water-clock,

! consisting of a cylindrical tube,, conical at bottom : the cylinderi was graduated, or marked out with divisions, to which the top ofI the water becoming successively contiguous, as it trickled out, the| vertex of the cone pointed out the hour.

HYDROSTATIC BALANCE, an instrument for finding thespecific gravities of bodies, both liquid and solid. It is also an.engine for extinguishing fires. See Balance and Hydrosta-tics.

HYDROSTATICS.

HYDROSTATICS, [from water, and ravuoi, the art ofweighing ,3 the science which treats of the gravity and equilibriumot non-elastic fluids. The term is also frequently applied to tbescience which explains the natme and properties of fluids in ge-neral, particularly their pressure, gravity, equilibrium, mid mo-tions: and lienee it includes Hydraulics and Pneumatics. How-ever, since this Rst treats on a distinct kind of fluids, via. elastic-fluids, it is more properly considered apart. Non-elastic fluids,

which make the object of Hydrostatics,, are generally denominat-ed liquids.

Op THE EQUILIBRIUM AND PRESSURE OF FLUIDS.

Bv Hydrostatics, properly so,called, in contradistinction fromVOL. III.—NO. toy.

Hydraulics, we are taught how to determine the gravity or pres-sure m fluids upon solids, or upon each other, in vessels u herewater is not allowed to escape or run off, but remains at rest. SirIsaac Newton defines a fluid to be a body whose parts are soloosely connected together, that they easily- yield to any force im-pressed upon them, and move freely amongst each other-. It isto be understood, that the force is applied to one part of the fluid,and not, equally on every part. In this sense,, fire, air, &c. areconsidered as fluids. In almost every physical speculation, whereexperiment can reach, the subject admits of some illustration;,where that is denied, the reasonings are in general vain and con-jectural. We do not know the form of the parts of which fluidsare composed, and can make no experiments to reduce them intoS then